Surgical resection of epileptogenic foci is a commonly practiced and often beneficial treatment for patients suffering debilitating seizures arising from otherwise intractable epilepsy. The success of this procedure depends on the ability of the medical team to precisely locate the epileptogenic zones in the patient's brain and to identify important cortical regions, such as eloquent cortex, that must be avoided during resection. To map epileptogenic activity, a combination of non-invasive imaging (e.g., magnetic resonance imaging or computer-aided tomography) and scalp recordings may be combined with electrocorticogram (ECoG) recordings from electrodes placed subdurally on the surface of the brain. A one- to two-week period of recording with subdural electrode arrays remains the accepted best clinical practice for localizing epileptogenic zones for the purpose of surgical resection. The electrode arrays also facilitate electrical stimulation of the cortex to locate important functional regions that must be preserved.
In general, to adequately identify the location of epileptogenic foci, a number of seizure events are recorded and the patient remains tethered to the recording equipment during the monitoring period. It is common practice to implant several subdural arrays to record from a wide area of the surface of the brain, and patients will typically have many cables exiting their heads. This invasive surgical procedure has drawbacks, including: (a) a lengthy and costly hospital stay is required; (b) a family member or sitter must typically be with the patient at all times; (c) patients may have difficulty coping with the need to remain in bed and tethered to the wall by cables; (d) long duration implantations increase the likelihood of serious infection; (e) there is a risk of intracerebral hemorrhage if the cables are accidentally pulled and the grids move; and (f) 10-20% of patients do not have enough seizures during their hospitalization to identify the epileptic zone with certainty.
These disadvantages of ECoG monitoring stem from the use of subdural arrays that are wired directly from within the patient's skull to external recording and data storage equipment. The risk of infection, created by the cables exiting the patient's head and the prohibitive expense of prolonged hospitalization, are generally the principal factors that limit the duration of ECoG monitoring to 1-2 weeks. It is also clinical practice to shorten the length of hospitalization by inducing seizures with anti-epileptic drug withdrawal since the patient, even with medically intractable epilepsy, will be taking medication to control seizures. Withdrawing medication is often associated with a change in the type of seizure experienced by the patient. The patient's usual or habitual seizure pattern may be replaced by more severe convulsive seizures that place the patient at risk for seizure-related morbidities of bodily trauma, aspiration, and hypoxia, as well as potentially fatal cardiac arrhythmias or cardio-respiratory arrest.
Seizures associated with drug withdrawal are also more likely to be non-habitual (atypical) and begin in brain areas that are otherwise well-controlled by medications. The recording of atypical seizure activity can result in false localization information.
While the use of electromagnetic telemetry to transmit brain activity wirelessly and, in particular, for recording epileptogenic activity is known, critical to the clinical application of wireless ECoG recording is the ability to electrically stimulate the cortex to determine regions of the brain responsible for functions that must be preserved during resection. In prior-art clinical practice, electrodes on wired subdural recording arrays are connected to an electrical stimulator using direct-wired, external mechanical connectors, and different combinations of electrodes on the arrays are stimulated electrically, usually with a series of charge-balanced biphasic current pulses. The patient's response to the stimulation determines whether the cortex underlying the electrodes is responsible for function that must be preserved. Since it is important to locate these functional regions as precisely as possible, the electrical stimulation is usually applied as a bipolar pulse train between adjacent pairs of electrodes on the subdural array. It may be necessary to perform these functional studies at various times throughout the monitoring period. There is therefore a need for systems capable of providing electrical stimulation at the discretion of the medical team at appropriate times during the monitoring period.